I've had several people ask: What exactly is a character arc? or How do I create a character arc?
Well, there are two types of character arcs: narrative (what happens to the character or around the character) and emotional arc (inner changes, within the character). They're equally important, but a lot of writers (especially fantasy writers) tend to neglect the emotional arc.
With every action, every scene, something needs to change to move the plot forward. Change is vital to a novel, especially when it's loaded with conflict. But your character has to change, too. One example from Connecting Emotional and Narrative Arcs is: If Jamie watches his Dad beat Red in a bowling tournament, how does Jamie feel about Red? Is he sympathetic, picturing himself in the situation of a painful loss? Or is he scornful of Red for choking at an important moment? (By the way, I highly recommend you read this article. It's super short but has great tips.)
If the charcter doesn't change, then what's the point? Your main character needs to feel some sort of emotion, have some sort of reaction, or change his or her way of thinking with everything that happens in your story. Otherwise, your character will wind up passive, boring, and/or unrelatable.
If you want a breakdown of narrative arc, try this article. It analyzes a picture book and shows page-by-page how the story progresses. A great blueprint.